In an internal combustion engine such as a spark-ignition engine or a diesel engine, the quantity of NOx contained in exhaust gas discharged from an exhaust passage to the outside (hereinafter may be referred to as “NOx discharge quantity”) must be reduced. An effective way of reducing the NOx discharge quantity is lowering the highest flame temperature (highest combustion temperature) through, for example, increasing the quantity of EGR gas circulated by means of an EGR apparatus, or delaying fuel injection timing.
However, when the quantity of EGR gas is increased in order to reduce the NOx discharge quantity, in the case of a diesel engine, the generation quantity of particulate matter (PM) increases. When fuel injection timing is delayed in order to reduce the NOx discharge quantity, fuel efficiency deteriorates.
Accordingly, in order to minimize the NOx discharge quantity in consideration of suppression of an increase in the discharge quantity of particulate matter (PM) and suppression of deterioration in fuel efficiency, the NOx discharge quantity is desirably controlled to a predetermined target value corresponding to the operating conditions of the engine. In order to accurately control the NOx discharge quantity to a predetermined target value, the NOx discharge quantity must be accurately estimated.
For such accurate estimation, a control apparatus for an internal combustion engine disclosed in Japanese Patent Application Laid-Open (kokai) No. 2002-371893 detects combustion pressure and intake-gas oxygen concentration by use of a cylinder pressure sensor and an intake-gas oxygen concentration sensor, and estimates the quantity of NOx generated as a result of combustion (hereinafter referred to as “combustion-generated NOx quantity”) on the basis of combustion temperature and gas mixture concentration calculated on the basis of the combustion pressure and the intake-gas oxygen concentration, wherein the estimation is performed by use of the extended Zeldovich mechanism, which is a typical known combustion model. Then, EGR gas quantity, fuel injection timing, or the like is controlled so that the estimated combustion-generated NOx quantity coincides with a predetermined target combustion-generated NOx quantity, whereby the NOx discharge quantity is controlled to a predetermined target value.
Incidentally, in an internal combustion engine equipped with an EGR apparatus, NOx contained in EGR gas is circulated into a combustion chamber via the EGR apparatus. In addition, the above-mentioned combustion-generated NOx quantity is the quantity of NOx generated in the region which is a portion of the combustion chamber and in which combustion occurs (hereinafter referred to as “combustion region”). Accordingly, in the remaining portion of the combustion chamber (hereinafter referred to as “non-combustion region”), the circulated NOx remains after combustion. Therefore, in order to accurately estimate the NOx discharge quantity, not only the combustion-generated NOx quantity but also the “quantity of NOx remaining in the non-combustion region” must be taken into consideration.
However, in the above-mentioned conventional apparatus, the “quantity of NOx remaining in the non-combustion region” is not taken into consideration at all. Accordingly, when the conventional apparatus is applied to an internal combustion engine equipped with an EGR apparatus, NOx discharge quantity cannot be accurately estimated, and thus, the NOx discharge quantity cannot be accurately controlled to a predetermined target value.